Liquid chromatograph pump and control method therefor

ABSTRACT

In a pump for feeding a liquid for chromatography, first and second plungers form first and second volumes respectively together with first and second chambers, the first plunger is prevented from changing the first volume sufficiently for feeding the liquid between the first and second volumes when the second plunger decreases the second volume to discharge the fluid from the second volume to an outlet path of the pump for the chromatography.

BACKGROUND OF THE INVENTION

The present invention relates to a pump for feeding a liquid forchromatography and a control method therefor.

In a prior art pump as disclosed JP-U-63-36668, when a first plungermove to put a liquid into the pump, a second plunger move to feed theliquid out of the pump so that a change in flow rate of the liquiddischarged from the pump is decreased.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a pump for feeding aliquid for chromatography and a control method therefor, by which pumpand method a minimal flow rate of the liquid is correctly maintainedwithout a discontinuation of the correctly maintained minimal flow rateof the liquid.

In a pump for feeding a liquid for chromatography, comprising first andsecond chambers, first and second plungers forming first and secondvolumes respectively together with the first and second chambers andbeing capable of reciprocating in the first and second chambersrespectively in such a manner that the first and second volumes arevariable in accordance with respective movements of the first and secondplungers, a communication path connecting fluidly the first and secondvolumes to each other, an inlet path communicating fluidly with thefirst volume, from which inlet path the liquid to be fed is capable ofbeing supplied into the first volume, and an outlet path communicatingfluidly with the second volume, by which outlet path the liquid to befed is capable of being discharged from the second volume, according tothe present invention, the first plunger is prevented from changing thefirst volume sufficiently for feeding the liquid between the firstvolume and the second volume when the second plunger decreases thesecond volume to discharge the fluid from the second volume to theoutlet path for the chromatography.

Since the first plunger is prevented from changing the first volumesufficiently for feeding the liquid between the first volume and thesecond volume when the second plunger decreases the second volume todischarge the fluid from the second volume to the outlet path for thechromatography, the minimal flow rate of the liquid is correctlymaintained without the discontinuation of the correctly maintainedminimal flow rate of the liquid.

It is further preferable for correctly maintaining the minimal flow rateof the liquid without the discontinuation of the correctly maintainedminimal flow rate of the liquid that the first plunger is prevented fromchanging the first volume sufficiently for changing a pressure of theliquid in the second volume through the communication path when thesecond plunger decreases the second volume to discharge the fluid fromthe second volume to the outlet path for the chromatography.

It is preferable for correctly keeping the pressure in the secondchamber stably constant that the communication path connects fluidly thefirst and second volumes to each other in series so that the liquid tobe fed for the chromatography is capable of being fed from the firstvolume through the communication path to the second volume, thecommunication path includes a check valve for allowing the liquid toflow from the first volume into the second volume when a pressure in thefirst volume is higher than a pressure in the second volume andpreventing the liquid from flowing from the second volume into the firstvolume when the pressure in the first volume is not higher than thepressure in the second volume. It is preferable for restraining thediscontinuation of the correctly maintained minimal flow rate of theliquid caused by the first plunger movement for decreasing the firstvolume to compensate the second plunger return movement increasing thesecond volume that when the second plunger decreases the second volumeto discharge the fluid from the second volume to the outlet path for thechromatography, the first plunger is capable of decreasing the firstvolume to increase the pressure of the liquid in the first volume tomore than a pressure in the inlet path (at an upstream side of a checkvalve in the inlet path) and is prevented from decreasing the firstvolume sufficiently for increasing the pressure of the liquid in thefirst volume to more than the pressure in the second chamber. The checkvalve may allow the liquid to flow from the first volume into the secondvolume when the pressure in the first volume is more than the pressurein the second volume and a difference in absolute value between thepressure in the first volume and the pressure in the second volume ismore than a predetermined value more than zero, and prevent the liquidfrom flowing from the first volume into the second volume when thepressure in the first volume is not more than a total amount of thepressure in the second volume and the predetermined value. It ispreferable for restraining the discontinuation of the correctlymaintained minimal flow rate of the liquid caused by the first plungermovement for decreasing the first volume to compensate the secondplunger return movement increasing the second volume that the firstplunger is prevented from decreasing the first volume sufficiently forincreasing the pressure of the liquid in the first volume to more thanthe total amount of the pressure in the second volume and thepredetermined value when the second plunger decreases the second volumeto discharge the fluid from the second volume to the outlet path for thechromatography. It is preferable for restraining the discontinuation ofthe correctly maintained minimal flow rate of the liquid caused by thefirst plunger movement for decreasing the first volume to compensate thesecond plunger return movement increasing the second volume that whenthe second plunger decreases the second volume to discharge the fluidfrom the second volume to the outlet path for the chromatography, thefirst plunger is capable of decreasing the first volume to increase thepressure of the liquid in the first volume to more than a pressure inthe inlet path (at an upstream side of a check valve in the inlet path)and is prevented from decreasing the first volume sufficiently forincreasing the pressure of the liquid in the first volume to more thanthe total amount of the pressure in the second volume and thepredetermined value. When the second plunger decreases the second volumeto discharge the fluid from the second volume to the outlet path for thechromatography, the first plunger may decrease the first volume toincrease the pressure of the liquid in the first volume to more than thepressure in the second volume and is prevented from decreasing the firstvolume sufficiently for increasing the pressure of the liquid in thefirst volume to more than the total amount of the pressure in the secondvolume and the predetermined value.

When the second plunger increases the second volume, the first plungerdecreases the first volume sufficiently for feeding the liquid from thefirst volume into the second volume so that the flow of the liquiddischarged from the pump is maintained. The movement of at least one ofthe first and second plungers (that is, a relative movement between thefirst and second plungers) should be controlled in such a manner that adifference between a difference (in absolute value) between theincreasing rate (per time) of the second volume increased by the secondplunger and the decreasing rate (per time) of the volume of the firstvolume decreased by the first plunger and a predetermined degree is keptwithin an acceptable range. For keeping the flow rate of the liquiddischarged from the pump desirably, it is preferable that the decreasingrate (per time) in absolute value of the first volume decreased by thefirst plunger is more than the increasing rate (per time) in absolutevalue of the second volume increased by the second plunger, and themovement of at least one of the first and second plungers (that is, therelative movement between the first and second plungers) is controlledin such a manner that a difference between a difference in absolutevalue between an increasing rate (per time) of the second volumeincreased by the second plunger and a decreasing rate (per time) of thevolume of the first volume decreased by the first plunger and a desiredflow rate of the liquid to be discharged from the second volume to theoutlet path for the chromatography is kept within a predetermined range.

For rapidly discharging a gaseous bubble out of the pump and securelypreventing the discontinuation of the correctly maintained minimal flowrate of the liquid, it is preferable that a change rate (per time) inabsolute value of the first volume with respect to a movement velocityin absolute value of the first plunger is larger than a change rate (pertime) in absolute value of the second volume with respect to a movementvelocity in absolute value of the second plunger. For keeping theminimal flow rate of the liquid correctly, it is preferable that thefirst plunger is capable of being stationary at least temporarily whenthe second plunger decreases the second volume to discharge the fluidfrom the second volume to the outlet path for the chromatography.

The outlet path may include a drain valve openable to discharge a gasout of the outlet path through the drain valve to fill the outlet pathwith the liquid when the first plunger decreases the first volume, andclosable to discharge the fluid from the second volume out of the pumpthrough the outlet path for the chromatography when the second plungerdecreases the second volume. It is preferable for rapidly dischargingthe bubble out of the pump and securely compensating the second plungerreturn movement increasing the second volume that a maximum change ratein absolute value of the first volume obtainable in accordance with(with respect to) the movement of the first plunger is larger than amaximum change rate in absolute value of the second volume obtainable inaccordance with (with respect to) the movement of the second plunger,and a minimum change rate in absolute value of the first volumeobtainable in accordance with (with respect to) the movement of thefirst plunger is smaller than the maximum change rate in absolute valueof the second volume obtainable in accordance with (with respect to) themovement of the second plunger. For minimizing an adverse effect causedby the second plunger return movement increasing the second volume, thata movement velocity in absolute value of the second plunger forincreasing the second volume is larger than a movement velocity inabsolute value of the second plunger for decreasing the second volume todischarge the fluid from the second volume to the outlet path for thechromatography.

It is preferable for minimize the change of pressure in the secondchamber on switching from the decrease of the second volume to theincrease of the second chamber that the first plunger is capable ofdecreasing the first volume to increase a pressure in the first volumewhen the second plunger decreases the second volume, that is, beforeswitching from the decrease of the second volume to the increase of thesecond chamber, and/or that the first plunger is capable of decreasingthe first volume to pressurize the fluid in the first volume to apressure insufficient for feeding the liquid from the first volume intothe second volume and not more than a desired pressure of the liquid tobe discharged from the pump, when the second plunger decreases thesecond volume, that is, before switching from the decrease of the secondvolume to the increase of the second chamber.

A method for controlling a pump for feeding a liquid for chromatography,including, first and second chambers, first and second plungers formingfirst and second volumes respectively together with the first and secondchambers and being capable of reciprocating in the first and secondchambers respectively in such a manner that the first and second volumesare variable in accordance with respective movements of the first andsecond plungers, a communication path connecting fluidly the first andsecond volumes to each other, an inlet path communicating fluidly withthe first volume, from which inlet path the liquid to be fed is capableof being supplied into the first volume, and an outlet pathcommunicating fluidly with the second volume, by which outlet path theliquid to be fed is capable of being discharged from the second volume,comprises the steps of:

-   -   opening a drain valve in the outlet path to discharge a gas out        of the outlet path through the drain valve so that the outlet        path is filled with the liquid when the first plunger decreases        the first volume,    -   closing the drain valve when the second plunger decreases the        second volume to discharge the fluid from the second volume to        the outlet path for the chromatography, and    -   controlling at least one of the movement of the first plunger        for decreasing the first volume sufficiently for feeding the        liquid from the first volume into the second volume and the        movement of the second plunger for increasing the second volume        (that is, a relative movement between the first and second        plungers) in such a manner that a difference between a        difference in absolute value between an increasing rate (per        time) of the second volume increased by the second plunger and a        decreasing rate (per time) of the volume of the first volume        decreased by the first plunger and a desired flow rate of the        liquid to be discharged from the second volume to the outlet        path for the chromatography is kept within a predetermined        range.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partially cross-sectional view showing a pump of theinvention.

FIG. 2 is a diagram showing pairs of large pump flow rate ranges andsmall pump flow rate ranges of embodiments of the invention.

FIG. 3 includes diagrams showing a relationship among operations in apump of the invention, time proceeding and conditions in the pump.

FIG. 4 is includes diagrams showing a relationship among operations inanother pump of the invention, time proceeding and conditions in theanother pump.

FIG. 5 is a partially cross-sectional view showing a liquid mixing andfeeding system in which the pumps of the invention are used.

FIG. 6 is includes diagrams showing another relationship amongoperations in the pump, time proceeding and conditions in the pump.

FIG. 7 is a partially cross-sectional view showing a liquid mixing andfeeding system in which the pumps of the invention are used.

FIG. 8 is includes diagrams showing another relationship amongoperations in the pump, time proceeding and conditions in the pump.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment as shown in FIGS. 1-3, an inlet path 10, an outlet path11, a first chamber 12 and a second chamber 13 are formed in a pump body1. First and second plungers 2 and 3 are supported by slide bearings 7and 7′ respectively to be movable in the first and second chambers 12and 13 respectively. The inlet path 10 for supplying a fluid into thefirst chamber 12 includes an inlet check valve 4, and a communicationpath communicating fluidly with the first and second chambers 12 and 13includes a discharge check valve 5. In each of the check valves 4 and 5,a movable member is pressed against a valve seat by a spring so that afluidal flow from a fluid tank 51 into the first chamber 12 or from thefirst chamber 12 into the second chamber 13 is allowed when a fluidalpressure at an upstream side of the check valve is more than a totalamount of a fluidal pressure at a downstream side of the check valve anda fluidal pressure caused by the spring, and is prevented when thefluidal pressure at the upstream side of the check valve is not morethan the total amount of the fluidal pressure at the downstream side ofthe check valve and the fluidal pressure caused by the spring. The firstplunger 2 is driven linearly by a rotational motor 21 through areduction gearbox 22 and a rotary-motion and linear motion converter 23,and the second plunger 3 is driven linearly by a rotational motor 21′through a reduction gearbox 22′ and a rotary-motion and linear motionconverter 23′. Seals 6 and 6′ hermetically seal volumes formedrespectively by a combination of the first plunger 2 and chamber 12 anda combination of the second plunger 3 and chamber 13 while the first andsecond plungers 2 and 3 are movable through the seal 6 and 6′respectively. A controller 50 generates control signals for the motors21 and 21′ on the basis of an output signal of a pressure sensor 60.

A solvent 51 is supplied into a pump through the inlet path 10, anddischarged from the outlet path 11 to be mixed with a specimen suppliedby an injector 53. A solution as a mixture of the solvent 51 and thespecimen proceeds through a column 54 so that the specimen is divided tocomponents thereof for componential analysis in a detector 55. Thecolumn 54 includes silica-gel particulate through which the solutionproceeds, so that a pressure loss of, for example, about 10 MPa isgenerated across the column 54.

A large (flow rate) pump is formed by the first chamber 12 and plunger 2and a driving mechanism therefor, and a small (flow rate) pump is formedby the second chamber 13 and plunger 3 and a driving mechanism therefor.

As shown in FIG. 2, the pump according to the invention may generate anextremely small flow rate of, for example, less than 1 mL/min or 1μL/min. A ratio between a maximum obtainable flow rate and a minimumobtainable flow rate in generally known pumps is 1:100. Therefore, thepump for the extremely small flow rate cannot generate a flow ratesufficient for filling rapidly with the fluid a fluidal line through thepump and an analysis system at the downstream side of the pump, and fordischarging rapidly gaseous bubble out of the pump. A desired flow rateis not obtainable correctly when the gaseous bubble exists in thefluidal line.

In the embodiments of the invention, the small pump generates theextremely small flow rate for analysis and the large pump generates thelarge flow rate for filling the fluidal line with the fluid and fordischarging the gaseous bubble out of the pump before the analysis.

The obtainable minimum flow rate of the large pump is smaller than theobtainable maximum flow rate of the small pump, and the obtainablemaximum flow rate of the large pump is larger than the obtainablemaximum flow rate of the small pump. The flow rate is determined as aproduct of a cross sectional area of the plunger and a velocity of theplunger. The total flow rate along an abscissa axis in FIG. 2 is a totalflow rate obtainable for a high pressure gradient pump operation inwhich a required maximum flow rate is from several dozen times toseveral hundred times of a required minimum flow rate. Therefore, arequired minimum limit of resolution in flow rate of the pump is lessthan one tenth or hundredth of the required maximum flow rate.

In an operation of the first embodiment as shown in FIG. 3, beforestarting the liquid feed of the extremely small flow rate for theanalysis, a drain valve 52 is opened and the first plunger 2reciprocates at a high frequency to generate a large flow rate of theliquid so that the gaseous bubble in the pump and the feel line isdischarged therefrom and the pump and feed line are filled with theliquid. Since the large pump is arranged at the upstream side of thesmall pump so that the liquid supplied from the inlet path 10 flows fromthe large pump into the small pump to be discharged from the outlet path11, the bubble can be easily discharged from the second chamber 2 to theoutlet path 11. Further, since the liquid is supplied into each of thefirst and second chambers 12 and 13 at the vicinity of the seal 6, 6′and is discharged from each of the first and second chambers 12 and 13at the vicinity of a stroke front end of the plunger 2, 3, the liquidflows without a flow stagnation through each of the first and secondchambers 12 and 13. When the first plunger reciprocates to discharge thebubble from the pump and feed line, the second plunger 3 is preventedfrom moving.

During the liquid feed of the extremely small flow rate for theanalysis, the drain valve 52 is closed, the first plunger. 2 isprevented from moving and the second plunger 3 moves at a low velocityto decrease the second volume so that the liquid is discharged from thepump at the extremely small flow rate. In response to that the secondplunger 3 reaches the stroke front end thereof, the second plunger 3returns to a stroke back end thereof at an obtainable maximum velocityand the first plunger 2 moves to decrease the first volume so that theliquid is discharged from the first chamber 12 into the second chamberto keep a constant flow rate to be discharged from the second chamber 13to the outlet path 11, that is, a total amount of a negative flow rateor volume increasing rate Q2 of the second chamber 13 and a flow rate orvolume decreasing rate Q3 of the first chamber 12 is kept at a desiredconstant flow rate Q1 to be discharged from the pump. Since theobtainable minimum flow rate of the large pump is smaller than theobtainable maximum flow rate of the small pump, the flow rate or volumedecreasing rate Q3 of the first chamber 12 can compensate the negativeflow rate or volume increasing rate Q2 of the second chamber 13 whilethe desired flow rate Q1 to be discharged from the pump is maintained,when the second plunger 3 returns to the stroke back end thereof. Sincethe second plunger 3 returns to the stroke back end thereof at theobtainable maximum velocity, the obtainable minimum flow rate of thelarge pump can be large so that the obtainable maximum flow rate of thelarge pump can be large.

It is preferable for increasing a pressure in the pump to a desiredpressure Pset that the first plunger 2 moves by Xini as shown FIG. 3 todecrease the first volume to pressurize the pressure in the pump to thedesired pressure. If the pressure in the pump is increased to thedesired pressure Pset by only the movement of the second plunger 3, thestroke of the second plunger 3 needs to be large and a time period forreaching the desired pressure Pset needs to be long.

In response to that the first plunger 2 reaches to the vicinity of thestroke front end thereof after a plurality of times of reciprocatingmovements of the second plunger 13 so that a distance or remainingavailable stroke between a position of the first plunger 2 and thestroke front end thereof becomes insufficient for keeping the desiredconstant flow rate Q1 to be discharged from the pump through the returnmovement of the second plunger 3 to the stroke back end thereof, thefirst plunger 2 returns to a stroke back end thereof while the secondplunger 3 moves to decrease the second chamber for keeping the desiredconstant flow rate Q1 to be discharged from the pump.

As shown in FIG. 4, the first plunger 2 may return to the stroke backend thereof at each forward stroke of the second plunger 3 in whichstroke the second plunger 3 moves to decrease the second chamber forkeeping the desired constant flow rate Q1 to be discharged from thepump. In this case, a distance between the stroke front and back ends ofthe first plunger 2 may be small and the first volume may be small. Whenthe second plunger 3 decreases the second volume to discharge the liquidout of the second volume just before the second plunger 3 returns towardthe stroke back end thereof to increase the second volume, the firstplunger 2 moves to decrease the first volume so that a difference inpressure between the first and second chamber is decreased to minimize achange of pressure in the second chamber.

As shown in FIG. 5, a high-pressure gradient system for changing amixing ratio between solutions A and B gradually stepwise by adjusting aratio between flow rates Qa and Qb of the solutions A and B while atotal amount of the flow rates Qa and Qb is kept constant may be formeda pair of the pumps of the invention. The ratio between the flow ratesQa and Qb of the solutions A and B is changed from 1:99 to 99:1, asshown in FIG. 6. When the total amount Of the flow rates Qa and Qb is 1μL/min, a correctly obtainable minimum flow rate, or minimum limit ofresolution of each of the flow rates Qa and Qb is 10 nL/min of onehundredth of 1 μL/min is required as a correctly obtainable minimum flowrate, or minimum limit of resolution of each of the flow rates Qa andQb. If the flow rate discharged from the pump is kept constant, anoutlet pressure of the liquid discharged from the outlet path 11 changesin accordance with a change of the mixing ratio, because a fluidalresistance of the column 54 changes in accordance with the change of themixing ratio.

A relationship between the mixing ratio and the outlet pressure of theliquid obtained when the flow rate discharged from the pump is keptconstant is measurable experimentally. Therefore, the change of theoutlet pressure of the liquid or a desired outlet pressure of the liquidin accordance with the change of the mixing ratio necessary when theflow rate discharged from the pump is kept constant can be estimated.When an actual outlet pressure of the pump is adjusted by a feedbackcontrol at the desired outlet pressure of the liquid necessary when theflow rate discharged from the pump is kept constant, the flow ratedischarged from the pump is kept correctly constant. As shown in FIG. 5,an output signal of a pressure sensor 60 a or 60 b corresponding to theactual outlet pressures of the pumps is input to a main controller 70 sothat the pump controllers 60 and 60′ controls the pumps to generate thedesired outlet pressure of the liquid.

When the actual outlet pressure of the pump is lower than the desiredoutlet pressure of the liquid, that is, an actual total amount of theflow rates Qa and Qb is lower than the desired total amount of the flowrates Qa and Qb, feedback gains for controlling the flow rates Qa and Qbrespectively are adjusted in accordance with the ratio between the flowrates Qa and Qb. For example, when the ratio between the flow rates Qaand Qb is 20:80, the feedback gain for the flow rate Qa is (20/100)×K(numerical constant), and the feedback gain for the flow rate Qb is(80/100)×K. When the actual total amount of the flow rates Qa and Qb issmaller than the desired total amount of the flow rates Qa and Qb by 5,an ordered value for the flow rate Qa is 20+(20/100)×K×5, and an orderedvalue for the flow rate Qb is 80+(80/100)×K×5.

Since the outlet pressure of each of the pumps varies in accordance witha time proceeding, the pressure in the first volume of each of the pumpsneeds to change in accordance with the change of the outlet pressure ofeach of the pumps. In order to prevent the liquid from flowing from thesecond volume into the first volume, the movement of the first plunger12 is controlled by the feed back control on the basis of comparisonbetween each of the pressures measured by the sensors 60 a′ and 60 b′and the pressure measured by the sensor 60 a in such a manner that thepressure in the first volume of each of the pumps is not more than thepressure in the second volume of the each of the pumps.

As shown in FIG. 7, the check valve 4 may be replaced by a shutoff valve58 in the inlet path 10, and the check valve 5 may be arranged in theoutlet path 11 instead of the communication path. In this embodiment, asshown in FIG. 8, the first plunger 2 does not compensate the increase ofthe second volume by the return stroke of the second plunger 3 towardthe back stroke end thereof to keep the pressure in the outlet path 11constant during the return stroke of the second plunger 3. Beforefeeding the liquid for the analysis, the first plunger 2 is positionedat the front stroke end thereof and the second plunger 3 is positionedat the back stroke end thereof while the shutoff valve 58 and the drainvalve 52 are opened. Thereafter, the first plunger 2 is returned to theback stroke end thereof to take the solution 51 into the first chamberwhile the second plunger 3 is stationary. After the first plunger 2reaches the back stroke end thereof, the shutoff valve 58 is closed.Subsequently, the first plunger 2 is moved toward the front stroke endthereof to decrease the first volume so that the gaseous bubble isdischarged out of the pump from the drain valve 52. Before the firstplunger 2 reaches the front stroke end thereof, the drain valve 52 isclosed and the first plunger 2 is stopped. Subsequently, the firstplunger 2 moves by the distance Xini to decrease the first volume sothat the liquid in the first and second volumes is pressurized to thepredetermined pressure Pset. Whie the first plunger is kept stationary,the second plunger 3 moves toward the front stroke end thereof todecrease the second volume to feed the liquid at the flow rate Q1.

The second plunger 3 may be driven to change the second volume by apiezoelectric actuator or a thermal expansion metallic actuator. Thefirst and second chambers 12 and 13 may be arranged in respective pumpbodies separated from each other and fluidly connected to each other bya communication pipe line.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A pump for feeding a liquid for chromatography, comprising, first and second chambers, first and second plungers forming first and second volumes respectively together with the first and second chambers and being capable of reciprocating in the first and second chambers respectively in such a manner that the first and second volumes are variable in accordance with respective movements of the first and second plungers, a communication path connecting fluidly the first and second volumes to each other, an inlet path communicating fluidly with the first volume, from which inlet path the liquid to be fed is capable of being supplied into the first volume, and an outlet path communicating fluidly with the second volume, by which outlet path the liquid to be fed is capable of being discharged from the second volume, wherein the first plunger is prevented from changing the first volume sufficiently for feeding the liquid between the first volume and the second volume when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography.
 2. A pump according to claim 1, wherein the first plunger is prevented from changing the first volume sufficiently for changing a pressure of the liquid in the second volume through the communication path when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography.
 3. A pump according to claim 1, wherein the communication path connects fluidly the first and second volumes to each other in series so that the liquid to be fed for the chromatography is capable of being fed from the first volume through the communication path to the second volume, the communication path includes a check valve for allowing the liquid to flow from the first volume into the second volume when a pressure in the first volume is higher than a pressure in the second volume and preventing the liquid from flowing from the second volume into the first volume when the pressure in the first volume is not higher than the pressure in the second volume.
 4. A pump according to claim 3, wherein the check valve is capable of allowing the liquid to flow from the first volume into the second volume when the pressure in the first volume is higher than the pressure in the second volume and a difference in absolute value between the pressure in the first volume and the pressure in the second volume is more than a predetermined value more than zero, and of preventing the liquid from flowing from the first volume into the second volume when the pressure in the first volume is not higher than a total amount of the pressure in the second volume and the predetermined value.
 5. A pump according to claim 4, wherein the first plunger is prevented from decreasing the first volume sufficiently for increasing the pressure of the liquid in the first volume to more than the total amount of the pressure in the second volume and the predetermined value when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography.
 6. A pump according to claim 5, wherein when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography, the first plunger is capable of decreasing the first volume to increase the pressure of the liquid in the first volume to more than a pressure in the inlet path and is prevented from decreasing the first volume sufficiently for increasing the pressure of the liquid in the first volume to more than the total amount of the pressure in the second volume and the predetermined value.
 7. A pump according to claim 6, wherein when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography, the first plunger is capable of decreasing the first volume to increase the pressure of the liquid in the first volume to more than the pressure in the second volume and is prevented from decreasing the first volume sufficiently for increasing the pressure of the liquid in the first volume to more than the total amount of the pressure in the second volume and the predetermined value.
 8. A pump according to claim 3, wherein when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography, the first plunger is capable of decreasing the first volume to increase the pressure of the liquid in the first volume to more than a pressure in the inlet path and is prevented from decreasing the first volume sufficiently for increasing the pressure of the liquid in the first volume to more than the pressure in the second chamber.
 9. A pump according to claim 8, wherein the check valve is capable of allowing the liquid to flow from the first volume into the second volume when the pressure in the first volume is more than the pressure in the second volume and a difference in absolute value between the pressure in the first volume and the pressure in the second volume is more than a predetermined value more than zero, and prevents the liquid from flowing from the first volume into the second volume when the pressure in the first volume is not more than a total amount of the pressure in the second volume and the predetermined value.
 10. A pump according to claim 1, wherein when the second plunger increases the second volume, the first plunger is capable of decreasing the first volume sufficiently for feeding the liquid from the first volume into the second volume.
 11. A pump according to claim 10, wherein a difference between a difference between the increasing rate of the second volume increased by the second plunger and the decreasing rate of the volume of the first volume decreased by the first plunger and a predetermined degree is kept within an acceptable range.
 12. A pump according to claim 10, wherein a decreasing rate in absolute value of the first volume decreased by the first plunger is more than an increasing rate in absolute value of the second volume increased by the second plunger, and the movement of at least one of the first and second plungers is controlled in such a manner that a difference between a difference in absolute value between an increasing rate of the second volume increased by the second plunger and a decreasing rate of the volume of the first volume decreased by the first plunger and a desired flow rate of the liquid to be discharged from the second volume to the outlet path for the chromatography is kept within a predetermined range.
 13. A pump according to claim 1, wherein a change rate in absolute value of the first volume with respect to a movement velocity in absolute value of the first plunger is larger than a change rate in absolute value of the second volume with respect to a movement velocity in absolute value of the second plunger.
 14. A pump according to claim 1, wherein the first plunger is capable of being stationary at least temporarily when the second plunger decreases the second volume to discharge the fluid from the second volume to the outlet path for the chromatography.
 15. A pump according to claim 1, wherein the outlet path includes a drain valve openable to discharge a gas out of the outlet path through the drain valve to fill the outlet path with the liquid when the first plunger decreases the first volume, and closable to discharge the fluid from the second volume out of the pump through the outlet path for the chromatography when the second plunger decreases the second volume.
 16. A pump according to claim 1, wherein a maximum change rate in absolute value of the first volume obtainable in accordance with the movement of the first plunger is larger than a maximum change rate in absolute value of the second volume obtainable in accordance with the movement of the second plunger, and a minimum change rate in absolute value of the first volume obtainable in accordance with the movement of the first plunger is smaller than the maximum change rate in absolute value of the second volume obtainable in accordance with the movement of the second plunger.
 17. A pump according to claim 1, wherein a movement velocity in absolute value of the second plunger for increasing the second volume is larger than a movement velocity in absolute value of the second plunger for decreasing the second volume to discharge the fluid from the second volume to the outlet path for the chromatography.
 18. A pump according to claim 1, wherein the first plunger is capable of decreasing the first volume to increase a pressure in the first volume when the second plunger decreases the second volume.
 19. A pump according to claim 1, wherein the first plunger is capable of decreasing the first volume to pressurize the fluid in the first volume to a pressure insufficient for feeding the liquid from the first volume into the second volume and not more than a desired pressure of the liquid to be discharged from the pump, when the second plunger decreases the second volume. 